Alterations in thyroid hormone status have been associated with a number of metabolic abnormalities including alterations in resting metabolic rate, exercise capacity and cardiac function. This study is a prospective protocol designed to examine changes in energy expenditure in patients with overt and subclinical hyperthyroidism and hypothyroidism compared to healthy controls. Patients with subclinical disease have been thought to have a milder form of thyroid dysfunction compared to patients with overt thyroid disease. One aim of this study is to determine whether alterations in metabolism occur in patients with mild or subclinical thyroid disease compared to normal controls and patients with severe thyroid function abnormalities. Additionally, this study aims to determine whether normalization of thyroid function results in return of metabolic parameters to values observed in control patients. Patients are recruited from Endocrinology and General Internal Medicine clinics at the University of Colorado Health Sciences Center. At the current time, data has only been analyzed for the subgroup of patients with either subclinical or overt hyperthyroidism in comparison to normal controls. All patients were admitted overnight for whole body indirect calorimetry to measure 24 hour energy expenditure and exercise capacity testing. On average, all patients were in energy balance during the calorimetry stay and no significant group differences were observed in this measure. Fat free mass measured by DEXA did not differ between groups. Hyperthyroid patients demonstrated significantly higher 24 hour energy expenditures than both subclinical hyperthyroid patients and controls. There was a significant positive correlation between free thyroid hormone serum levels (Free T4) and 24 hour energy expenditure. Hyperthyroid patients expended an average of 759 kcal/d and subclinical hyperthyroid patients expended 243.9 kcal/d more than controls. The 24EE, when adjusted for fat free mass, was significantly higher in hyperthyroid patients compared to subclinical hyperthyroid patients and controls. Exercise capacity was also assessed in all patients. Subclinical hyperthyroid and overt hyperthyroid patients demonstrated reduced exercise capacity compared to controls. A subset of subclinical hyperthyroid patients (n=9) were retested after normalization of their thyroid function. Measurements of body mass index, fat free mass, and 24 hour energy expenditure improved, but due to the small number of patients did not reach statistical significance. In conclusion, subclinical and overt hyperthyroidism induced elevations in 24 hour and sleeping energy expenditure. Subclinical hyperthyroidism patients have reduced exercise capacity compared to controls. These data suggest a continuum of metabolic derangements in patients with excess thyroid hormone and patients with mild or subclinical hyperthyroidism have measurable changes in energy expenditure and exercise capacity.